[llvm-dev] Mem2reg: load before single store

[Johannes Doerfert via llvm-dev]

On 9/14/20 9:30 AM, James Y Knight via llvm-dev wrote:
> On Mon, Sep 14, 2020 at 3:19 AM László Radnai via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> A problem arises, and I am not sure if it is really a problem or just
>> weird C-compliant behavior.
>>
>> int a; // or, equally, int a=0;
>>
>> int main(){
>>   int b;
>>   if (b) // (*)
>>
> At this line, you invoke undefined behavior by reading the value of "b",
> before it's been initialized. At this point, the compiler may do whatever
> it likes.

FTR, it is *not* UB to read the value of b, you will read undef which is 
totally fine. It is however UB to branch on undef.
At that point we can stop discussing what "happens next".

~ Johannes



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